Making an startup is a question of investing time and money. So the first thing you have to keep in mind is how much money you would like to make at least from that startup. That is the exit goal. Lets say you want to make 1 million in 2 years (note: 1 million in 2 years is a key progress indicator - KPI). In order to reach that goal you need to plan how much you should be earning or how many paying customer you need to have every certain time. Those are your milestones. Lets say for instance that in order to get your expected goal, you need 100.000 paying customers in 6 months. 6 months has past. How many customers did you get? If you got 80.000 (that is 20% less), you need to readjust your goals or become more aggressive. But if you got 10.000, then you should leave the startup because you will not reach your goal and that means you are wasting your time.
In order to put the exit goal and the milestones, just follow the "book" instructions and plan well.
Taking this decision by guts or intuition is something very wrong, because in an startup there will be always up and down moments. Follow the plan, measure, and take decisions based on your milestones and KPIs.